This invention relates generally to method and apparatus for determining gas transmission characteristics of materials, and more particularly, the gas permeability characteristics of barrier materials.
The invention is specifically applicable to and will be described with particular reference to an instrument for measuring oxygen and water vapor permeability of barrier materials for electronic displays such as organic light emitting diodes and field emission displays requiring highly sensitive permeation measurements. However, those skilled in the art, while recognizing the benefits of the invention for precisely determining gas transmission characteristics in a sensitive application, will also recognize that the invention is not necessarily limited to barrier materials but could be applied to any permeable material where the permeability of the material is desired to be determined in a short time, nor is the invention necessarily limited to detecting elemental or simple compound gases, but could have application for detecting transmission characteristics of hydrocarbons or gaseous organic compounds.
The food industry has long recognized that the shelf life of food is correlated to the solubility and diffusivity characteristics of food packaging material. It is well known that native (i.e., uncoated) plastics are very permeable to water and oxygen. Testing standards have long been established to determine permeability characteristics of packaging material. Barrier coatings have been developed to retard permeability of oxygen and water vapor through the plastics. It is to be recognized that the shelf life of food is typically measured in days. For example, 30 days is generally regarded as an acceptable shelf life for pharmaceuticals or packaged consumerable goods, such as potato chips.
In marked contrast, displays, such as television screens or flat panel displays on a laptop computer, have a significantly longer operating life. For example, display requirements typically specify that the brightness of the display will be at least 50% of its original brightness after 10,000 hours of use. However, for displays which are degraded by trace amounts of oxygen or water vapor, a serious failure rate will occur at much less than the expected 10,000 hour lifetime.
Today""s display devices require fabrication using plastic substrates because plastic is lightweight, impact resistant, and can produce glass light transmission characteristics. Particularly, microelectronic devices, organic light emitting devices (OLED), and field emission display (FED) are being developed for flat display panel applications as well as electronic encapsulation packaging. Because of the high sensitivity of these materials to oxygen and water, especially formulated barrier coatings, sometimes termed engineered plastic substrates, are being developed. The barrier coatings are thin film barriers, typically consisting of metal (AL) or oxide (SiO2, AL2O3) having layer thickness of only about 20 to 30 nanometers and applied to plastic substrates by vacuum deposition techniques. A plurality of thin, specially formulated barrier films are applied to flexible plastic webs, films, or sheets significantly decreasing permeability of barrier coated plastics.
Uncoated plastics, whose properties are otherwise appropriate for panel displays, have permeabilities for water in the vicinity of 10 g/m2/day. Experimental estimates have been made which suggest that a desirable goal of about 10xe2x88x926 g/m2/day for water and 10xe2x88x926 cc/m2/day for oxygen is desired as a design permeation goal for plastic barrier materials used in panel display applications under discussion. Currently, within the trade, the best commercially available instrument for measuring gas permeabilities has a sensitivity limit of about 10xe2x88x923 g/m2/day for water and about 10xe2x88x923 cc/m2/day for oxygen. Reference can be had to an article entitled xe2x80x9cThin Film Technologyxe2x80x9d, pages 20-24, in the October, 2000 issue of Vacuum Technology and Coating in which target permeation rates at this level are specified with maximum sensitivity of current measuring instruments cited as being limited to the order of 0.005 cc/m2/day. For reference and comparison purposes, converting the sensitivity achieved by the prior art of 0.005 cc/m2/day to standard cc""s/second/sample (i.e., 0.005 cc""sxc3x9710,000/areaxc3x9786,400/day) yields a published, prior art maximum sensitivity of 4,320,000 cc""s/second/cm2. In any event, currently available instruments do not have the sensitivity needed to determine the suitability of the specially developed engineered plastic substrates for display panel applications.
An article published by Modern Controls Inc., entitled xe2x80x9cMeasuring Oxygen Permeability through Today""s Packaging Barriersxe2x80x9d, by Robert L. Demorest, describes the testing procedures commonly used in the packaging industry for measuring oxygen permeability, or more correctly, oxygen transmission rates. In the article, it is noted that prior to 1975, ASTM specification D-1434 was commonly used. In the ASTM D-1434 test, the plastic sample is mounted in a gas transmission cell to form a sealed semi-barrier between two chambers. One chamber contains the test gas at a specific high pressure and the other chamber, at a lower pressure, receives the permeating gas. Two procedures are set forth. In one procedure, the lower pressure chamber is maintained near atmospheric pressure and the transmission of the gas through the test specimen is indicated by a change in volume. In the second test procedure, the lower pressure chamber is initially evacuated and the transmission of the gas through the test specimen is indicated by an increase in pressure as measured by a manometer. Specifically, an initial vacuum is pulled in both chambers. The vacuum lines are closed and one of the chambers is flushed with a test gas. After some time (in the hours) has passed to achieve a steady state condition, the manometer is read over a series of time intervals to determine permeance, etc. The vacuum test is a static, not dynamic, test because the lines are closed. This gives a coarse instrument which is slow in response. Thus, the pressure test method or Dow Cell technique, is a static determination of the change of pressure in the test chamber after steady state conditions have been established from which information about the permeability of the plastic is obtained. The time of the test takes hours and the test is destructive. The sensitivity is limited. The Modern Controls article noted that the test was seldom capable of testing oxygen barriers below 1.00 cc/m2/day.
The current method in wide use today was published by ASTM as specification D-3985, in 1981. In this method, the sample to be tested is clamped between two chambers as discussed above, but, one side is exposed to a gently flowing oxygen stream while the other chamber is exposed to a nitrogen stream. As oxygen molecules permeate through the sample into the nitrogen stream, they are picked up and carried into a coulometric sensor. This sensor causes a release of 4 electrons electrochemically for every oxygen molecule which passes through it. The electrons form a current passed through a resistor creating a voltage which can be recorded. Such tests are also subject to lengthy time periods to achieve steady state conditions and the measurements are lacking in sensitivity because the permeant gas is only fractionally present in the sample.
In the literature, a paper entitled xe2x80x9cComparative Study of Oxygen Permeation Through Polymers and Gas Barrier Filmsxe2x80x9d, 2000 Society of Vacuum Coaters, presented Apr. 15-20, 2000, at the 43rd Annual Technical Conference Proceedings, discussed four different methods which were experimented with to determine oxygen transmission rates in barrier films. The four methods investigated included Oxtran, which is the standard method used to measure oxygen with the coulometric sensor discussed above. The second method was described as a time lag method which is generally similar to the D-1434 ASTM spec. In the time lag method, the sample sheet is clamped between two vessels evacuated to a pressure of 1xc3x9710xe2x88x924 Pa. Gas was then bled into the upper vessel at atmospheric pressure and the pressure in the lower vessel was monitored as a function of time using a Penning gauge. As in the ASTM spec., once steady state conditions were reached, the pressure increase per time is proportional to the gas transmission. The third method was described as a GTR10 method which clamped the film into the two chamber arrangement discussed above. A vacuum vessel was provided adjacent the space under the film and the space and vacuum vessel were evacuated to 1xc3x9710xe2x88x924 pa. A feeding side of the membrane was then exposed to a gas at 1-3 atmospheres and the gas permeating through the semi permeable barrier was collected in the vacuum vessel. After a set time, the vacuum vessel was cut off from the gas supply and the collected gas streams passed through to a chromatograph which measures the heat conductivity to determine the amount of gas. The fourth test procedure was described as a mass spectrometric method. In this method, a gas cell was placed in a UHV system (10xe2x88x9210 mbar) to face the entrance aperture of a mass spectrometer. The gas cell contained a fixed volume of gas that had an exposed diameter of 4 mm which was covered by the barrier specimen. The partial pressure of the permeant gas was then measured as a function of time and from the time constant of drop of partial pressure, the gas transmission was calculated.
The article stated that the partial pressure versus time curve measured by the mass spectrometer had insufficient slope; that going to smaller gas transmission rates made measurements by the mass spectrometer difficult and concluded that the mass spectrometer had a relatively big margin of error because of its small sample size.
Within the patent literature there is disclosed an outgasing technique in U.S. Pat. No. 5,591,898 and further refined, for continuous sensing application, in U.S. Pat. No. 6,009,743. In this technique, the plastic barrier saturated with a test gas and the outgasing of a plastic is measured to construct a degassing rate of decay curve which is correlated to permeability, etc. The test arrangement is the two chamber type with a neutral gas containing the out gas detected by a conventional sensor, such as an oxygen sensor or the like. Because the permeant gas is diluted by the neutral gas, there are limits to the sensitivity of this approach.
U.S. Pat. No. 4,944,180 discloses a somewhat conventional two chamber test box arrangement with a neutral or carrier gas flowing through one chamber and a permeant gas flowing at positive pressure through the other chamber. A long interface tube connects the chamber receiving permanent gas transmitted through the test specimen to a mass spectrometer. This arrangement has a potential for sensitive measurements because of the sensitivity of the mass spectrometer. However, the test chambers are at positive pressure and the transmitted permeant gas is diluted by the neutral or carrier gas which renders its use for low atomic numbered gases difficult. In addition, a capillary interface is required.
In summary, the prior art has developed and is developing barrier materials needed for long life in applications which are extremely sensitive to select gases or vapors, such as oxygen and water vapor. The multi-layer barrier materials under discussion are required to have such reduced permeability or low transmission rates that instruments available today or discussed in the prior art do not have sufficient sensitivity to allow meaningful testing of the new barrier materials. In addition, most, if not all, of the prior art systems discussed above establish permeability and gas transmission rates on a relative scale. That is, if material xe2x80x9cAxe2x80x9d exhibits a first reading and material xe2x80x9cBxe2x80x9d exhibits a second higher reading, then permeability of material xe2x80x9cBxe2x80x9d can be established as a function of the permeability of material xe2x80x9cAxe2x80x9d. Calibration can then be established by setting a base material xe2x80x9cAxe2x80x9d from which other measurements can be compared. Calibration as an absolute or standardized value does not occur. In addition, most of the tests require that an equilibrium be established on both sides of the barrier material before meaningful measurements can be taken which increases the time of the test. To some extent, test time is reduced by measuring the out gassing characteristics of the material. However, the material must still be saturated to some extent with the permeant gas before the measurements can be taken. Still further, many of the test systems discussed are destructive in nature and do not permit non-invasive sampling of production barrier material.
Thus, it is one of the main objectives of the present invention to provide a system, method, and apparatus, which can measure the permeability of barrier materials with a higher degree of sensitivity than that of currently available instruments. This feature, by itself or in combination with other objects of the invention discussed below, forms one of the underpinnings of the invention.
This object along with other features of the invention is achieved in a system for measuring transmission of a selected gas, vapor, or aroma of interest through a barrier material to establish permeability which, in the preferred embodiment, includes a test box having first and second facing surfaces confronting one another with a continuous seal extending from one of the facing surfaces to circumscribe a sealable area. A clamp mechanism is actuable from an open position to a clamped or closed position whereat the first facing surface contacts one side of the barrier material to form a test gas chamber extending from one side of the material and a seal in the second facing surface sealing engages the opposite side of the barrier material to form a sealed measurement chamber extending from the opposite side of the material. Importantly, the sealable area bounded by the seal in the second facing surface defines a measurement sealable area that spans a distance sufficient to permit, as a function of the barrier characteristics of the barrier material, a uniform diffusion of the gas under a xe2x80x9chardxe2x80x9d vacuum through the barrier material into the measurement chamber. Preferably, a continuous seal is provided for the test gas chamber so that both measurement and test gas chambers are sealed.
The system includes the test gas chamber having an inlet port connected to a source of the gas of interest, an outlet port, and a flow valve connected to a source of vacuum for controlling flow of the gas of interest into and out of the test gas chamber. A second vacuum system is provided in valved communication with the measurement chamber for drawing a vacuum of at least about 5xc3x9710xe2x88x924 Torr in the measurement chamber when actuated (In practice, a pressure less than this value can be established. However, a pressure of at least this value can be readily obtained and will produce superior instrument results when compared to the prior art.) A mass spectrometer in fluid communication with the measurement chamber is provided for analyzing the concentration of the gas of interest diffused into the measurement chamber at any given time. A mechanism is provided for controlling the mass spectrometer, the vacuum pump and vacuum system, and the flow valve to permit sampling of the concentration of the gas in the measurement chamber at set intervals, whereby the differential in pressure between the vacuum in the test chamber and measurement chamber is set to be sufficient to permit diffusion of the selected gas through the barrier material without permanently distorting the barrier material while establishing sufficiently high vacuum levels for the mass spectrometer to analyze the partial pressure of the selected gas transmitted to the measurement chamber to achieve highly sensitive and accurate measurements.
A significant feature of the invention resides in the fact that the mass spectrometer functions by continuously pumping the measurement chamber at a high vacuum to determine the content of a gas or vapor of interest at any selected time while the test gas chamber is continuously circulating a fresh supply of a gas or vapor of interest to the test gas chamber. The dynamic conditions on both sides of the barrier material allow the barrier material to achieve saturation or equilibrium in an optimally fast time when contrasted to prior art static test procedures. Additionally, in the preferred embodiment, a high vacuum, constant speed (550 l/sec) turbomolecular pump produces a high vacuum (low pressure) resulting in a good signal-to-noise ratio. Accurate determination of gas transmission rates not only at saturation or equilibrium, but also during the time the barrier material is transitioning from an unsaturated to a saturated state is possible. Significantly, because accurate measurements can be taken during the transition time, any number of known predictive modeling techniques can be used to predict equilibrium or establish pass/fail criteria so that testing does not have to extend to the time whereat equilibrium is achieved.
In accordance with another feature of the invention, the system includes a roughing vacuum pump valved to initially draw a vacuum in both the test chamber and measurement chamber, a molecular pump backed with a roughing pump valved to draw a hard vacuum in the measurement chamber and the control mechanism insures the integrity of the test gas chamber and measurement chamber before drawing and after drawing a hard vacuum in the measurement chamber whereby the measurements obtained from the mass spectrometer are assured as to accuracy.
In accordance with another aspect of the invention, the system includes in the preferred embodiment, as a source for the vapor of interest, a container for water, a heater for heating the water to a temperature of approximately 20 to 75 degrees C. (a temperature range known to be conducive to produce vapor under vacuum although not essential for formation of water vapor to practice the invention), and an agitator to produce water bubbles when the container is subjected to a rough vacuum from the rough vacuum pump whereby water vapor is produced and the transmission of water vapor is correlated by the control mechanism to the life of the barrier material at relative humidity levels.
In accordance with yet another object of the invention, the gas of interest is selected preferably to have an atomic mass of 50 or less which is selected because of the ability of gases within this range to diffuse or permeate rapidly through micro-cracks and/or pin holes in barrier materials of the type now being applied to OLEDs, FEDs, etc. (In accordance with the broader inventive scope, the gas of interest can include organics such as aromatics and the mass is limited by the sensitivity of the mass spectrometer which can extend to a gas of interest having an atomic mass of 200 or less.)
In accordance with another aspect of the invention, the chambers are fitted with heaters whereby the excitation of the gas of interest and its transmissibility through the barrier material is increased to reduce the test time.
In accordance with another specific object of the invention, the gas of interest other than oxygen and water vapor includes an inert gas, preferably helium and the control mechanism correlates the transmissibility of helium through the barrier material to the traditional gases of interest, such as oxygen, and humidity whereby a non-invasive measurement is obtained and the measurement is calibrated to a NIST standard to assure an absolute value.
In accordance with yet another feature of the invention, the system includes a plurality of seals establishing a like plurality of pairs of measurement and test gas chambers longitudinally spaced from one another with each test gas chamber in each pair of chambers having its inlet port in valved fluid communication with a common gas manifold and its outlet port in fluid communication with a common exhaust manifold. The exhaust manifold is valved to an atmospheric vent and to the roughing vacuum pump and the gas manifold is in valved communication with a plurality of source gases whereby one or more of the source gases may be valved into fluid communication from the common manifold with select test gas chambers while the rate of flow of one or more of the source gases into each test gas chamber may be individually set by the control mechanism so that a number of gases of interest may be simultaneously tested and/or the differential pressures in chamber pairs for a select gas of interest may be varied to produce even faster test times and/or multiple gases of interest may be evaluated in combination in select chamber pairs.
In accordance with yet another object of the invention, the barrier material in one embodiment of the invention is in a roll form and the system has a payout reel at one longitudinal end thereof and a take-up reel at the opposite longitudinal end and the control mechanism includes a programmed routine implemented by a computer for synchronizing the rotation of the reels to sequentially move the roll longitudinal set distances relative to the box after a plurality of the test gas and measurement chamber pairs have simultaneously analyzed the gas transmission characteristics of the selective gases of interest over a segment of the roll whereby the production barrier material may be non-invasively tested with test data recorded for quality control purposes. This technique may be incorporated in an on-line or post production system for testing. Other implementations of this method could use an in-line or conveyor transport system for sheet material. A hybrid system which uses roll to roll handling, then sheets the material, and handles the sheets, is also suitable for production testing.
In accordance with a specific but important feature of the invention, the system of the invention may optionally include a porous support for supporting the barrier material against excessive deflection into the measurement chamber, preferably including a grid having a lattice structure lacking sharp edges whereby differential vacuum levels of a sufficient level may be maintained between the test gas chamber and the measurement chamber without damaging the barrier material. Because the grid is positioned in the measurement chamber which has less pressure than the test gas pressure, test gas flow into the measurement chamber is not impeded should the barrier material contact the grid and special calibration techniques are not necessary.
In accordance with another feature of the invention, a method for determining the transmission characteristics of at least one gas or vapor through plastic barrier materials of the type used for encapsulating electronic displays including organic light emitting diodes and field emission displays is provided which includes the steps of:
a) forming a vacuum across the barrier material by establishing a sealed test gas chamber extending from one side of the barrier material and a sealed measurement chamber extending from the opposite side of the barrier material and the sealed chambers extend over a surface area of the material of at least about 10 cm2 whereby sufficient material area is provided to assure average gas transmission characteristics through the barrier material;
b) establishing a rough vacuum across the barrier material between the measurement and test gas chambers;
c) establishing a high vacuum in the measurement chamber;
d) measuring the background partial pressure of the measurement chamber under said high vacuum;
e) continuously flowing the gas of interest through the test gas chamber; and
f) continuously measuring partial pressure of the gas of interest in the measurement chamber over time to correlate the change in partial pressure to the permeability of said material. With this method, fast testing times inherently occur because saturation of the barrier material by the gas of interest is not necessary to establish transmissibility characteristics of the gas of interest through the barrier material.
In accordance with a specifically important feature of the invention, the partial pressure of the measurement chamber is measured by a mass spectrometer having a sensitivity of at least about 2xc3x9710xe2x88x924 Torr. The mass spectrometer is calibrated to a NIST standard whereby the partial pressure measurements recorded by the mass spectrometer during testing establish absolute transmission test values.
In accordance with another important aspect of the invention, full testing of a barrier material can occur by allowing the test to proceed to an equilibrium condition whereat the flow of a test gas through the barrier material is constant. The sensitivity of the inventive instrument provides for the desired accuracy to permit evaluation of the barrier material. However, the sensitivity of the instrument allows for an accurate transmission determination of the gas of interest through the barrier material during the entire test time and prior to the sample reaching equilibrium. Because the transmission graph is accurately determined throughout the time it takes for the gas or vapor of interest to reach equilibrium, the invention contemplates the use of any known curve fitting or statistical analysis methodologies to establish pass/fail criteria prior to the time the gas transmission reaches equilibrium. Accordingly, the test time in the sense of at least pass/fail can be significantly reduced.
In accordance with another aspect of the invention, which is somewhat distinct, but related to several inventive features, the gas of interest is chosen as helium. The system uses a mass spectrometer calibrated with helium to establish an absolute value when the mass spectrometer is used to measure the gas transmission characteristics of helium. This value, in turn, is correlated to any gas of interest. More specifically, the instrument is operated in a normal manner to establish a transmission rate reference curve for any desired gas of interest and a correlation between the transmission graphs is established (such as that resulting from the superposition of one graph over the other graph) to determine if test specimens meet pass/fail criteria for any given gas of interest on the basis of sensing helium transmission through the specimen. In fact, several gases of interest can be correlated on the basis of a single helium gas transmission test. Because of the light weight of helium, the transmission of helium through the barrier material will quickly establish equilibrium, or a discernible trend to equilibrium to further reduce test time. Because the mass spectrometer is calibrated with helium to an absolute standard (NIST), the measurement, although a correlation, is based on an absolute and not relative measurement values. Finally, helium is a non-invasive gas allowing it to be used for testing on production samples without saturating the material or sample with a gas that is damaging to the display life.
In summary, some of the significant objectives of the present invention is the provision of a system for measuring gas transmission rates (permeability) of a gas/vapor or gases/vapors of interest through a permeable material having one or more or any combination of the following:
a) dynamic testing by continuously flowing test gas through test gas chamber in an instrument capable of measuring gas transmission by continuously drawing and analyzing gas transmitted through the barrier material to a measurement chamber and present a more accurate and more responsive test measurement than possible in static tests;
b) suitable for quantitative testing of engineered plastic substrates, i.e., barrier coated plastic substrates, as well as other permeable materials;
c) sensitivities that can exceed 10xe2x88x926 cc/m2/day (as high as 10xe2x88x928 cc/m2/day) for a number of gases of interest and/or 10xe2x88x926 g/m2/day of water vapor;
d) measurements that can be correlated to an NIST standard and therefore indicative of an absolute measurement;
e) fast test times;
f) potential for non-invasive or non-destructive material testing so that the instrument can be used to continuously measure and record data for production barrier material;
g) simultaneous measurement of a plurality of singular gases of interest or a plurality of gases of interest or a singular gas of interest at multiple stations at set varying conditions for a reduction in test time;
h) ability to determine the rate of transmission of any number of gases of interest thereby allowing for potential of developing correlations between any given gas to any other gas of interest to minimize testing time;
i) ability to separately determine humidity affects on the material thereby avoiding prior art limitations which have to set a controlled humidity level at which the material is tested for permeability;
j) separately control excitation activity of a gas of interest by heat to further reduce test time; and/or
k) reduce test time by partial pressure measurement via a mass spectrometer which can be taken before the material reaches saturation limits.
These and other objects, features, and advantages of the present invention will suggest themselves to those skilled in the art upon reading and understanding the Detailed Description of the Invention set forth below.